Document error codes in API spec

[yamcodes]

Clear and concise description of the problem

The possible error responses from each endpoint are not documented within the OpenAPI spec.

Suggested solution

Document all the possible error responses from each endpoint.

Alternative

No response

Additional context

No response

Validations

• Read the Contributing Guide.
• Read the README.md.
• Check that there isn't already an issue that requests the same feature.